The present invention relates to a method for manufacturing a primary preform for optical fibres using an internal vapour deposition process, comprising the steps of:                i) providing a hollow glass substrate tube having a supply side and a discharge side,        ii) surrounding at least part of the hollow glass substrate tube by a furnace set at a temperature T,        iii) supplying doped or undoped glass-forming gases to the interior of the hollow glass substrate tube via the supply side thereof        iv) creating a reaction zone with conditions such that deposition of glass on the inner surface of the hollow glass substrate tube will take place, and        v) moving the reaction zone back and forth along the length of the hollow glass substrate tube between a reversal point located near the supply side and a reversal point located near the discharge side of the hollow substrate tube so as to form at least one preform layer on the inner surface of the hollow glass substrate tube, which at least one preform layer comprises several glass layers.        
A method as described in the introduction is known per se from U.S. Pat. No. 4,741,747. More in particular, the aforesaid patent discloses a method of fabricating optical preforms according to the PCVD method, wherein glass layers are deposited by moving a plasma back and forth between two points of reversal inside a glass tube whilst adding a reactive gas mixture to the tube at a temperature between 1100° C. and 1300° C. and a pressure between 1 and 30 hPa. The regions of nonconstant deposition geometry at the ends of the optical preform are reduced by moving the plasma nonlinearly with time in the area of at least one reversal point.
U.S. Pat. No. 4,608,070 relates to a method and a device for manufacturing optical preforms wherein a furnace is placed over a rotating substrate tube, wherein the temperature setting of the furnace is a function of r, viz. the radial distance, and x, viz. a longitudinal position along the length of the substrate tube. The temperature function mentioned in said U.S. patent only applies to radial and longitudinal distances and is set at a constant value during the entire deposition process.
U.S. Pat. No. 4,659,353 relates to a method for manufacturing optical fibres, wherein silica layers having a constant thickness but varying dopant percentages are deposited in the interior of a substrate tube, using MCVD technology, in which use is made of a heat source having a circular, asymmetrical temperature profile.
U.S. patent application US 2004/0173584 relates to a method for manufacturing an optical preform, using MCVD technology, wherein the dimension of a plasma flame is controlled as a function of the dimension of the preform.
Japanese publication JP 2004-036910 relates to a quartz burner adapted to effect a wide and uniform high temperature zone at a focal position of the flame by mixing oxyhydrogen gas.
U.S. patent application US 2005/0144983 relates to a method for manufacturing a preform, using CVD technology, wherein the temperature of at least one of the heating element of the furnace and the glass substrate tube is measured, after which the amount of heat generation of the heating element is adjusted based on the measured temperature.
U.S. Pat. No. 4,740,225 relates to a method for manufacturing optical preforms wherein a special temperature profile is set so that core layers are applied to the interior of the substrate tube in a desired thickness.
An optical fibre consists of a core and an outer layer surrounding said core, also referred to as cladding. The core usually has a higher refractive index, so that light can be transported through the optical fibre. The core of an optical fibre may consist of one or more concentric layers, each having a specific thickness and a specific refractive index or a specific refractive index gradient in radial direction.
An optical fibre having a core consisting of one or more concentric layers having a constant refractive index in radial direction is also referred to as a step-index optical fibre. The difference between the refractive index of a concentric layer and the refractive index of the cladding can be expressed in a so-called delta value, indicated Δi% and can be calculated according to the formula below:
            Δ      i        ⁢    %    =                    n        i        2            -              n        cl        2                    2      ⁢              n        i        2                            where:        ni=refractive index value of layer i        ncl=refractive index value of the cladding        
An optical fibre can also be manufactured in such a manner that a core having a so-called gradient index refractive index profile is obtained. Such a radial refractive index profile is defined both with a delta value Δ% and with a so-called alpha value α. To determine the Δ% value, use is made of the maximum refractive index in the core. The alpha value can be determined by means of the formula below:
      n    ⁡          (      r      )        =                    n        1            ⁡              (                  1          -                      2            ⁢            Δ            ⁢            %            ⁢                                          (                                  r                  a                                )                            α                                      )                    1      2                      where:        n1=refractive index value in the centre of het fibre        a=radius of the gradient index core [μm]        a=alpha value        r=radial position in the fibre [μm]        
A representation of the refractive index as a function of the radial position in an optical fibre is referred to as a radial refractive index profile. Likewise it is possible to graphically represent the refractive index difference with the cladding as a function of the radial position in the optical fibre, which can also be regarded as a radial refractive index profile.
The form of the radial refractive index profile, and in particular the thicknesses of the concentric layers and the refractive index or the refractive index gradient in the radial direction of the core determine the optical properties of the optical fibre.
A primary preform comprises one or more preform layers which form the basis for the one or more concentric layers of the core and/or part of the cladding of the optical fibre that can be obtained from a final preform. A preform layer is built up of a number of glass layers.
A final preform as referred to herein is a preform from which an optical fibre is made by using a fibre drawing process.
To obtain a final preform, a primary preform is externally provided with an additional layer of glass, which additional layer of glass comprises the cladding or part of the cladding. Said additional layer of glass can be directly applied to the primary preform. It is also possible to place the primary preform in an already formed glass tube, also referred to as “jacket”. Said jacket may be contracted onto the primary preform. Finally, a primary preform may comprise both the core and the cladding of an optical fibre, so that there is no need to apply an additional layer of glass. A primary preform is in that case identical to a final preform. A radial refractive index profile can be measured on a primary preform and/or on a final preform, with the radial refractive index profile of the final preform corresponding to the radial refractive index profile of the optical fibre.
The length and the diameter of the final preform determine the maximum length of optical fibre that can be obtained from the final preform.
To decrease the production costs of optical fibres and/or increase the output per primary preform, the object is to produce, on the basis of a final preform, a maximum length of optical fibre that meets the required quality standards.
Accordingly there is wish to increase the amount of additional glass that is applied to a primary preform.
The diameter of a final preform can be increased by applying a thicker layer of additional glass to a primary preform. Because the optical properties of an optical fibre are determined by the radial refractive index profile, the thickness of the layer of additional glass must at all times be in the correct proportion to the layer thickness of the preform layers of the primary preform that will form the core, more in particular the one or more concentric layers of the core in the optical fibre. Consequently, the layer thickness of the glass layer additionally applied to the primary preform is limited by the thickness of the preform layers being formed by means of the internal vapour deposition process.
In other words, the following criterion must be met:
            CSA              CL        ,        vezel                    CSA              i        ,        vezel              =            CSA              CL        ,        vv                    CSA              i        ,        vv                            where:        CSACL,fibre=Cross-sectional area of the cladding in the fibre        CSACL,vv=Cross-sectional area of the cladding in the final preform        CSAi,fibre=Cross-sectional area of het concentric layer i in the fibre        CSAi,vv=Cross-sectional area of the preform layer i in the final preform        
An increase of the diameter of the final preform leads to an increase of the cross-sectional area of the cladding. From the above criterion it follows that the cross-sectional area of the preform layer or layers (CSAi, vv) must also be increased. The cross-sectional area of said one or more preform layers must thus be increased during the internal vapour deposition process. This means that, given an unchanged diameter of the hollow glass substrate tube, the thickness of the preform layers being deposited on the inner surface of the hollow glass substrate tube must be increased.
The present inventors have surprisingly found that when relatively thick preform layers for step-index type optical fibres are deposited, the refractive index of a doped preform layer is not sufficiently constant in radial direction. An insufficiently constant radial refractive index adversely affects the optical properties of the optical fibre. In particular factors such as dispersion, cutoff wavelength, bending losses and attenuation may be affected.
The present inventors have also found that when relatively thick preform layers for gradient index type optical fibres are deposited, the alpha value is different from the alpha value being aimed at. A deviating alpha value adversely affects the properties, in particular the bandwidth, of the gradient index optical fibre.